The recent development of light weight rechargeable lithium batteries, with the capability to store comparatively large amounts of energy in a relatively small package, has made lithium batteries the power source of choice for portable electronic devices. Once manufactured, the life span of a lithium battery is largely determined by the type of use and by the recharge approaches used. While the use of the lithium ion battery is determined primarily by the consumer, the type of recharging can be determined by the device manufacturer and/or by the charger manufacturer. Although the battery manufacturer can supply recharging guidelines, these guidelines typically prescribe fairly conservative charge rates in order to keep any detrimental battery charging effects at a minimum. This leads to battery recharge times in the 2 to 4 hour range or even greater times. Consumers naturally prefer to charge portable devices in much shorter times in many cases, such as in less than 1 hour. Thus there is a conflict between the conservative charging guidelines written to ensure a long battery life, and the actual practices and preferences in the use of the batteries in the field. Improper rapid charging of lithium battery packs can greatly shorten the battery life, and in worst case scenarios, can lead to battery pack swelling or rupture of a battery pack and under certain circumstances, rapid charging can also create a possible fire risk.
A battery's capacity, described as the “C” rating, is defined as the sum of a constant current that a rated battery cell can deliver during a 20 hour discharge cycle and still remain within the rated voltage limits at room temperature. For example, an ideal battery that can produce 50 mA of current for 20 hours while staying within the prescribed voltage window would have a C rating of 50 mA×20 H=1000 mAh or 1 Ah. A “perfect” battery would be 100% efficient during charge and discharge. If a perfect battery had a C rating of 1 Ah, then it could deliver the 1 A for 1 hour, or 0.5 A for 2 hours, or 100 mA for 10 hours, or 50 mA in 20 hours, and so on. In addition, a perfect battery with a C rating of 1 Ah could be recharged in 1 hour by charging at 1 A for 1 hour, or 0.5 A in 2 hours, or 100 mA in 10 hours. Other terms commonly used in the description of a battery's charge and battery capacity are the state-of-charge (SOC) and the depth-of-discharge (DOD). SOC and DOD both have units of % and are complimentary definitions. SOC indicates the relative amount of energy stored in a battery compared to its fully charged state. DOD indicates how much of the battery's energy has been used compared to its fully charged state. For example, for a perfect 1 Ah battery, when drained of 0.25 Ah the battery would have a SOC of 75% and a DOD of 25%, and if drained of 0.5 Ah the battery would have a SOC of 50% and a DOD of 50%. The voltage of a battery is often specified as the open-cell-voltage (OCV) where the cell voltage is measured without external load. And the full-charge-capacity (FCC) is the capacity of the cell with a full charge at any given point in its life. Using the example of the perfect battery described above when new, the FCC would be 1 Ah. As the battery ages, the FCC is reduced.
While battery capacity can be arbitrarily increased or decreased with the addition/reduction of battery material, the voltage characteristic of a given battery chemistry remains independent of the battery capacity. For this reason, the discharge and recharge of a cell is commonly discussed in terms of the C rating. For example, if the perfect 1 Ah battery is discharged at a maximum rate of 1 C, then the maximum discharge rate would be 1 A. The same perfect battery, charged at a maximum rate of 2 C would see a maximum discharge current of 2 A.
In a rechargeable lithium ion battery, energy is stored and retrieved in a chemical reaction via the migration of lithium ions between electrode pairs. The positive electrode (cathode) and the negative electrode (anode) are both able to bind lithium ions. During discharge, the lithium ions move from the anode to the cathode, releasing energy in the process. During a charge cycle, an electric field created between the cathode and anode forces the lithium ions back to the anode, absorbing energy in the process. Under some circumstances, including high C rate charging, a small portion of lithium ions form metallic lithium and are deposited on the anode during the recharge phase. This lithium material partially reacts with electrolyte and is then no longer available for charge storage and as a result the capacity of the battery is diminished. This phenomenon is referred to as “lithium plating.” The lithium plating reduces the capacity and the life of the battery.
In conventional prior known approaches, a prior known approach to battery charging methodology has two phases, a constant current or “CC” phase, followed by a constant voltage or “CV” phase. During the CC phase, a current that is considered a safe current for the battery is applied until the cell voltage reaches a target voltage, for example, 4.2 Volts, and then the current is reduced while the voltage is maintained in the constant voltage phase, until the current falls to a minimum when charging stops. Known prior battery chargers also monitor conditions such as temperature and may prior known approaches may stop charging when temperature falls below a minimum such as 10 degrees C., or 0 degrees C.
FIG. 1A illustrates in a flow diagram a known prior approach dual stage constant current CC and constant voltage CV or “CC/CV” charge process; and FIG. 1B illustrates a graph of a typical battery charge cycle using the CC/CV process.
The flow diagram illustrated in FIG. 1A shows the steps used in a controller for operating the CC/CV charge process. Step 110 begins the process. At step 111, the charge process begins. The maximum current is applied to the battery at this step. At step 113, the constant current phase begins. During this phase the charge process charges the battery at a predetermined maximum current. The current may be the maximum available from the charger, or, a maximum current considered safe according to manufacturer specifications, such as 1 C. The process continues at step 113 and tests to see if a maximum voltage “Max V” has been reached. Until the maximum voltage is reached, the constant current charging continues by remaining at step 113.
When the test at step 113 indicates that the maximum voltage Max V has been reached, the charging process transitions to a “constant voltage” mode of operation at step 115. During the constant voltage charging process, the current into the battery is allowed to fall as the battery reaches full capacity. At step 117 the current flowing into the battery is tested to see if the cutoff limit has been reached (current into the battery is measured and found to be less than a minimum charging current) and when the cutoff is reached, the process transitions to step 119, the “stop” state.
In FIG. 1B, the left vertical axis of charging graph 120 depicts the charge voltage which is represented by data line 124. The right vertical axis depicts the charge current which is represented by data line 122. The units of the right axis are labeled as C indicating the C rating of the battery. The bottom axis is the charge time in hours. The two distinct phases of a CC/CV charger are indicated near the top of the graph with the CC phase as 130 and the CV stage as 132
The maximum charge rate in this example is 1 C as indicated by the peak value of the charge current line 122 in the constant current or CC phase 130. Following the current line 122 from the CC phase into the constant voltage of CV phase to the last current value in the CV phase 132, the cutoff current is approximately 0.15 C. The maximum charge voltage is seen by the value of voltage line 124 in the CV stage 132 and is approximately 4.2V. In this example charging case, the total charge time is about 1 hours and 10 minutes as seen by the termination of the charge current line 122.
To achieve lower recharge times, the current in the CC phase can be increased to 2 C or higher. However, it is known that the life and capacity of the lithium battery is greatly diminished when repeatedly charging at these higher rates compared to charging at lower rates, such as 0.5 C. Obviously, a charging system that is able to charge a battery in the least amount of time, without accelerating battery degradation, is desired.
FIG. 2 is a graph that illustrates cell capacity degradation with increasing C rate. The data depicted in graph 200 illustrates the detrimental effects that charging a battery at a higher C rate can have on the usable capacity of a battery cell as are described in an article titled “Factors that affect cycle-life and possible degradation mechanisms of a Li-ion cell based on LiCoO2”, Journal of Power Sources, Vol. 111, Issue 1, pp. 130-136, by S. S. Choi et. al. (2002). Referring to the graph of FIG. 2, it can be seen that when using a CC/CV charger with a charging rate as little as 1.4 C, the capacity of the cell illustrated in the graph has dropped to about 27% of its initial capacity in 500 charge cycles. The same chemistry cell, charged at a 1 C rate still retains about 800 mA after 500 cycles. As shown in the graph in FIG. 2, a higher constant current charging rate degrades a battery quickly and shortens battery life.
The open cell voltage (OCV) of a battery cell can be calculated as shown in Equation 1:Vcell=Vcathode−Vanode  EQUATION 1
Direct measurement of the battery voltages Vcathode and Vanode is typically done by inserting a 3rd reference electrode in the electrolyte region of the battery. Because of the delicate nature of the procedure, it is typically only performed in a laboratory setting by the battery manufacturer. When successful, the cathode and anode potentials are extracted from a battery with very few cycles and usually at room temperature. However these voltages are not available for a battery in actual use because there is no 3rd reference electrode.
Battery degradation is classified in two basic categories: active material loss and internal impedance increase. In the first category, active material loss, the loss of material reduces the available chemicals for the ionic processes needed to create current. Although battery manufacturers seal the batteries, there is still some electrolyte loss due to parasitic reactions inside the cell. A battery cell pack that is charged inappropriately can exhibit swelling due to the gas produced and the swelling can, in extreme cases, damage the device the battery pack is installed in.
In the second category, impedance increase, the usable amount of energy available for external loads is reduced by increases in the internal resistance. Increasing impedance increase is the prevalent degradation a lithium battery cell experiences. Internal resistance increases with age of the battery and with the number of charge/discharge cycles. Further, at lower temperatures, the internal resistance increases. When the internal impedance increases, lithium plating is more likely during charging as the increased internal impedance negatively affects internal potentials. Because of the impedance increase at lower temperatures, some prior known approach chargers include a cut off temperature sensor and will not charge below certain temperatures, like 10 degrees C. or 0 degrees C. However impedance increases also occur at higher temperatures than these and charging that is acceptable at room temperature may cause lithium plating at lower temperatures, particularly as the battery cells age.
Lithium plating causes both types of degradation, active material loss and internal impedance increase. When the lithium ions become lithium metal and react with the solvent, the number of lithium ions available to transport charge is reduced. In addition, as the lithium metal decomposition products accumulate at the anode, there are less locations for lithium ions to exchange their charge. Because of this dual degradation, it is desirable to avoid conditions that accelerate lithium plating. Lithium plating reduces capacity and life of the battery cells.
It is known that during charging of lithium ion battery cells, lithium plating occurs when the voltage drop across anode material causes the graphite surface potential at the anode of the lithium cells to fall below the lithium potential. FIG. 3 depicts in a graph 300 the limitations on charging presented by the lithium plating phenomenon. In graph 300, the vertical axis is a plot of the anode potential compared to the lithium potential, in volts. The horizontal axis illustrates the state of charge (SOC) in percent, for a charging cycle. At the left side of the graph, the SOC is at 0% and charging begins, the charging ends when the SOC is 100% representing a full charge. Trace 305 illustrates the graphite anode open cell voltage for a lithium ion cell. The OCV for the anode is greater than zero for the entire charging cycle. However, the trace 307 depicts the anode potential v. the lithium potential, For low temperature, or rapid charging, or for an aged battery cell, the internal resistance can increase, resulting in an anode potential that falls to zero or becomes negative compared to the lithium potential. This is shown inside the area 309 in FIG. 3, in this area, lithium plating can occur. Thus the lithium plating phenomenon is a limit on the rate of charge that can be achieved without damaging the battery. It is also known that the impedance of the battery changes with temperature. Because battery impedance increases at lower temperatures, charging at lower temperatures, for example when charging outdoors or in vehicles, can cause lithium plating to occur even at charging current levels that would be appropriate at room temperatures.
A charge methodology that avoids lithium plating and thus extends battery life while maintaining battery capacity is clearly desired. U.S. patent application Ser. No. 14/014,195, published Mar. 6, 2014, filed Aug. 29, 2013, titled “METHOD AND APPARATUS OF CHARGING THE BATTERY WITH GLOBALLY MINIMIZED INTEGRAL DEGRADATION POSSIBLE FOR PREDEFINED CHARGING DURATION,” naming Yevgen Barsukov et al. as inventors, which is co-owned with the present application, is hereby incorporated herein by reference in its entirety. In the above referenced patent application, improvements are made in reducing the recharge time of a lithium cell. In laboratory testing and modeling of specific battery chemistry, multiple charging profiles are developed to suit battery age, impedance, temperature and state of charge (SOC). With the lithium chemistry empirically characterized, a set of optimized constant current-constant voltage (CC/CV) charging profiles can be loaded into a battery charging apparatus. These charging profiles can then be used to reduce the overall charge time while avoiding charging conditions that degrade the battery, including degradation due to lithium plating at high charge rates. The approach described in the above referenced patent application is a step forward beyond previous known prior approaches towards the goal of fast battery recharge without additional cell degradation. This approach requires substantial computations offline in order to characterize each of the particular battery cells and to create the CC/CV charging profiles for each battery pack made, and if the material or battery cells are modified, these computations must be repeated and new profiles are required. However, further optimization of the charging methodology is desired to further reduce the charge time without incurring additional battery degradation.
Improvements are thus needed in battery chargers and in methods for charging lithium ion batteries. The avoidance of battery degradation during charging would maintain battery cell capacity at higher levels over a larger number of cycles while extending battery life. Damage to the battery cells and to the equipment in which the battery cells are installed due to improper charging could be avoided.